The head involution defective (hid) locus is a previously uncharacterized gene located on the left arm of chromosome III of Drosophila melanogaster. At present, eleven recessive lethal mutations of this gene have been recovered and studied. These investigations reveal that the hid gene must be expressed sometime during the first 12 hours of embryogenesis so that the head of the fly larva will form correctly. In addition to its embryonic role, hid must also be expressed during the pupal stage to facilitate the construction of the adult fly. The role of the hid gene during embryogenesis may be related to its role later in development, that is, in both cases the hid product is required in specific groups of cells to permit these cells to undergo the proper rearrangements during development. The genetic control of cell movement and tissue reorganization, two fundamental developmental processes, is poorly understood. Consequently, further studies on the role of the hid gene in Drosophila development may provide valuable insight into the genetic regulation of these processes and such knowledge would permit the construction of a model to guide future developmental investigations. At present, the cloning of hid DNA is in progress. The availability of the cloned DNA will permit: (1) a more precise determination of the timing and localization of hid expression during development, (2) eventually lead to the generation of monoclonal antibodies to the hid protein(s), and (3) facilitate attempts to identify other Drosophila genes which either are involved in the control of hid expression and/or share homologous DNA sequences which possess a common function in development.